Soft handoffs are one of the greatest advantages of Code Division Multiple Access (CDMA) technology over other multiple access technologies being used for wireless communication systems because of the smooth and seamless transition provided to mobile-telephones traversing from one cell to another cell. Generally, handoffs involve handling calls by a progression of base stations to keep the calls active as the mobile-telephones traverse from one cell to another cell. One type of handoff, referred to herein as soft handoff, is a state in which a mobile-telephone maintains simultaneous communication links with multiple base stations.
In wireless communication systems based on the well-known IS-95 standard for CDMA, a soft handoff occurs in the following manner. A mobile-telephone initiates a call via a primary base station, which is the base station in communication with and in control of call processing for the mobile-telephone. The primary base station is a part of a set of base stations referred to as an xe2x80x9cactive set,xe2x80x9d which are base stations with which the mobile-telephone is in communication. The active set includes the primary base station and, perhaps, one or more secondary base stations, wherein the secondary base stations are base stations in communication with but not in control of call processing for the mobile-telephone. When the call is first initiated, the active set includes only the primary base station.
After the call has been established, the mobile-telephone continuously measures signal quality of pilot signals transmitted from base stations which are likely candidates for a handoff. Such base stations are referred to herein as neighbor base stations, which are a part of a set of base stations referred to as a xe2x80x9cneighbor set.xe2x80x9d Soft handoff is initiated when a pilot signal transmitted from a neighbor base station is detected with a signal quality exceeding a threshold T_ADD. Such neighbor base station with a pilot signal quality exceeding T_ADD becomes a candidate for the active set, and is referred to herein as a candidate base station (which is a part of a xe2x80x9ccandidate setxe2x80x9d of base stations), wherein the threshold T_ADD indicates that traffic channels associated with the base station with a pilot signal quality above the threshold T_ADD could be successfully demodulated by the mobile-telephone. When a candidate base station is detected, the mobile-telephone transmits a pilot signal quality measurement message to the primary base station to indicate the measured pilot signal quality.
The candidate base station becomes a secondary base station and, thus, part of the active set, when the mobile-telephone processes a Handoff Direction Message transmitted from the primary base station listing the candidate base station. Once in the active set, the mobile-telephone may be in the soft handoff state and in communication with the secondary base station, i.e., former candidate base station. That is, in soft handoff, the active set base stations transmit the same information signal to the mobile-station and receive the same information signal from the mobile-station (although the information signal may be modulated with a different Walsh code).
The problem with soft handoff is that a pool of traffic channels need to be reserved at each base station for possible use in soft handoff. For example, an IS-95 based CDMA base station has a total of sixty-four communication channels. Of the sixty-four communication channels, fifty-five are traffic channels. Of the fifty-five traffic channels, sixteen traffic channels are reserved for use in soft handoffs. In other words, only thirty-nine traffic channels are available at any IS-95 based CDMA base station for assignment to mobiletelephones in a non-soft handoff situation. Thus, system capacity is degraded twenty-nine percent due to sixteen out of fifty-five traffic channels being reserved for use in soft handoffs. However, if the number of traffic channels reserved for use in soft handoffs is reduced, then the number of calls being dropped as a result of mobile-telephones traversing from one cell to another cell may increase due to traffic channels not being available for soft handoffs. Accordingly, there exists a need to improve system capacity without compromising soft handoff.
The present invention is a method for improving system capacity without compromising soft handoff by optimizing the number of communication links used in soft handoff. The number of communication links used in soft handoff are optimized by minimizing the number of base stations in the active sets based on signal characteristics of associated communication links, such as signal quality, signal strength and radio frequency (RF) performance. By minimizing the number of base stations in the active sets, the number of traffic channels reserved for soft handoffs may be reduced since less traffic channels would be needed for soft handoffs, thereby increasing system capacity by increasing the number of available traffic channels for assignment to mobile-telephones in non-soft handoff situations.